The present invention relates to a method for producing illusory magnetic stimulation.
The present invention relates to a method according to the preamble of claim 1.
Methods and apparatuses of the kind discussed herein are used for measuring and examining the response of biological tissue through stimulating the same by electro-magnetic means.
Using conventional techniques, it is possible to stimulate biological tissue such as the brain, the peripheral nervous system, muscles and the heart by virtue of inducing an electric field in the tissue. In magnetic stimulation, the induction of the electric field is provided by means of a changing magnetic field. Different types of apparatus constructions based on magnetic stimulation are described, e.g., in U.S. Pat. Nos. 4,940,453; 5,047,005; 5,061,234; 5,066,272 and 5,267,938 and FI Pat. No. 100,458.
Magnetic stimulators deliver the changing magnetic field by means of a coil. The coil transduces the electric energy fed by the power source of the stimulator into magnetic field energy. The coil can be an integral or separate part of a magnetic stimulator. Some practicable coil constructions are discussed, e.g., in U.S. Pat. Nos. 4,994,015; 5,078,674; 5,116,304 and 5,725,471.
Magnetic stimulation has been found a risk- and pain-free method of stimulating human brain, peripheral nervous system or muscles. The method has a plurality of applications in basic research, diagnosis and therapy.
Magnetic stimulation of the brain, however, also involves activation of skin on the skull. The electromagnetic field induced by the coil is weaker the greater the distance from the coil. Hence, when the electromagnetic field in the brain is brought sufficiently strong to excite the nerve cells, the field strength on the scalp is manyfold. This high-level excitation causes contraction of scalp muscles and activation of nerve ends on the skin, which is felt by the test person as a sensation resembling as a knock or pinch. The sensation may also be slightly painful if the stimulated area coincides with a greater mass of muscles. The sensations felt on the scalp cannot be eliminated by any conventional technique known in the art.
Magnetic stimulation by conventional techniques also includes a strong sonic bang that evokes an auditory sensation. This sound effect can be attenuated but not generally eliminated through the use of hearing protectors.
Both the stimulation of the scalp and the sonic bang emitted by the stimulation coil activate the sensory nerve paths leading to the brain and, thus, the brain areas associated with sensory information. This brain activation interferes with the magnetic stimulation of the brain evoked directly by the stimulating electromagnetic field. Therefore, it is often difficult to identify whether a given result of stimulation is caused by direct stimulation of the brain or evoked by the sensory feelings on the scalp and the sonic bang heard by the test person.
The contribution of scalp stimulation and the auditory evoked response related to the sonic bang emitted by the coil in the overall stimulated response can be estimated by producing fake pulses of magnetic stimulation and then measuring the effect of stimulation caused by these pulses. Herein, the term fake magnetic stimulation refers to stimulation that produces the same sensory and auditory stimulation as a real magnetic stimulation, yet inducing such a low field on the brain that cannot cause direct stimulation of nerve cells in the brain.
A typical test necessitating the use of fake magnetic stimulation is encountered in the examination of the brain of patients suffering from depression. Herein, it is impossible to tell in a reliable manner whether the detected effects are related to the multisensory response evoked by the magnetic stimulation or are they triggered by the stimulation of the brain tissue.
Fake magnetic stimulation by means of conventional techniques and methods is accomplished by moving or rotating the coil into different positions above the head so that the field induced over certain areas of the brain is diminished.
One problem hampering the conventional technique is that the coil must be moved or rotated for the fake stimulation, whereby switching between the real magnetic stimulation and the fake magnetic stimulation takes time from tens of seconds up to several minutes. Also the position of the coil in respect to the object being stimulated may change appreciably when moving or tilting the coil into the fake stimulation position and back therefrom.
Another problem of conventional technique is that the change of coil position and, thus, the change of stimulation method is relatively easy to detect by the test person.
It is an object of the invention to overcome the above-described disadvantages and to provide an entirely novel type of method and apparatus for producing fake magnetic stimulation.
The goal of the invention is achieved by producing the fake magnetic stimulation through the induction of at least two variable magnetic fields so that at least two of the magnetic fields are oriented in different directions, e.g., opposite to each other in the target area. The directions and magnitudes of the magnetic fields are set so as to attain partial cancellation of the electric fields, which are induced by the varying magnetic fields, within the target area such as the brain tissue, in a manner that avoids causing essential stimulation of the target area tissue by the direct effect of the fake stimulation field. The magnetic fields oriented in different directions can be produced using, e.g., two or a greater number of coils fed by varying currents. Alternatively, a suitable formed coil having, e.g., a figure-of-eight shape can be used. Notwithstanding the desired interior cancellation of the induced electromagnetic fields, the exterior field must be so strong as to attain stimulation of the exterior tissue such as the scalp possibly overlying the target area. The magnetic field for the real deep-stimulating effect can be induced using the same coils mentioned above or, alternatively, at least one of them. When using a plurality of coils, the stimulation is attained by feeding the coils with such varying currents that cause the electric fields induced by the coils to sum in the target area such as the brain tissue simultaneously exciting both the nerve cells of the target area and the possibly overlying tissue layer such as the scalp and the muscles associated therewith. Herein, both the real and the fake magnetic stimulation pulses can be issued without physically moving the coil or plurality of coils. Advantageously, the current pulses fed to the coils during both the fake and the real magnetic stimulation are kept essentially equal in order to maintain the sound level of the sonic bang emitted by the coils essentially unchanged during both of these stimulation operations. In a similar fashion, the invention is also applicable to the stimulation of the peripheral nervous system, muscles and spinal cord. Instead of man, a test animal for instance may be the subject of stimulation.
The response of the stimulating field on the actual target area such as the brain can be measured and assessed by subtracting the responses of the fake magnetic stimulation from those of the real magnetic stimulation.
The invention offers significant benefits.
By virtue of the invention, it is possible to issue the real and the fake magnetic stimulation pulses in an alternating manner without any need for moving or tilting the stimulator coil, thus facilitating a fast alternation of stimulation modes and keeping the position of the coil relative to the object stationary. Furthermore, the test person has no possibility of detecting a change of the stimulation mode from a movement of the coil.
In some types of conventional stimulator embodiments, the fake magnetic stimulation is achieved by moving the coil over another brain area, whereby a risk arises that the activation of a different brain area subjected to the fake magnetic pulse may affect the results of the test session. As compared with such prior-art embodiments, the present invention has the benefit of eliminating a direct effect of local stimulation on the brain and, instead, permits the direct stimulating effect of the fake magnetic pulse to be applied in a controlled and minimized manner.
In addition to these, the invention has other embodiments offering further benefits.
Advantageously, the invention can be applied using, e.g., electronic control of the stimulation current fed to the stimulator coils, whereby the operator of the stimulator can easily control the stimulator apparatus and select a desired mode of stimulation. The stimulation mode, whether the real or the fake magnetic stimulation, may be selected manually, e.g., by pressing a key or automatically in a computer-aided environment.
In a computer-aided control method of the stimulation mode, the invention may also be applied so that the stimulation mode is determined by an algorithm stored in a control computer or using a random sequence of selection, which allows an improved method of assessing the effect of magnetic stimulation applied as, e.g., a therapeutic treatment.